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I just noticed that Time Machine is backing up 15 GB (it's been running for a few hours; this means that Time Machine thinks that 15 GB of data changed in one hour which is absurd). I've only been doing some programming in the Terminal, and the usual web browsing/iTunes etc.

Why is this happening? Is there a way to see which files does Time Machine think changed this much? How to stop this behavior?

Edit: It just occured to me that the two times it happened I was using a VMWare Virtual Machine. Is it possible that Apple made the terrible oversight of not having a working binary diff and it's backing up the Virtual Machine image in its entirety every time?

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Yes, it is the whole virtual disk which gets saved, there is no binary diff.

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Back up software without a binary diff is insane. But thanks for the answer. – houbysoft Oct 1 '12 at 5:02
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To Time Machine a file is a file and if it is changed the file is copied again. This is required due to the - rather ingenious - way that hard links are used to have multiple backups use the same single file "on disk".

I found a reasonable solution to make virtual disks work well with Time Machine. Namely to make a sparse disk image and put the virtual machine over there. Then OS X automatically maintains a lot of single, small files comprising your virtual machine disk and only those actually affected needs to be backed up automatically.

See http://hints.macworld.com/article.php?story=20040625012304236 for instructions how to release claimed but unused space from the sparse disk image.

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I exclude my virtual machines from Time Machine entirely and rsync them manually to the backup disk. "If Time Machine backs up a running virtual machine, it attempts to capture files that are being written to. The resulting backups are likely corrupt and unusable." This would also apply to sparsebundles containing VMs. rsync is inconvenient but at least I know to only do it when my VMs are shut down. – gabedwrds Oct 2 '12 at 3:24
I spend most of my time outside Fusion with my virtual machines shut down. Hence Time Machine has plenty of time to catch up and it works well for me. For people with Fusion running all the time it would make good sense to do as you say. – Thorbjørn Ravn Andersen Oct 2 '12 at 6:38

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